The observed behavior is expected.
The instantiated cubes do not start with zero velocity because they are not instantiated from a prefab with zero velocity.
The "Instantiate" method on the Entity Manager creates a copy of the given entity along with its component values. To make every instantiated cube start with zero velocity, instantiate a prefab with zero velocity instead.
This can be easily achieved in the provided sample in two steps:
1) Drag and drop the cube game object into the assets folder to create a prefab.
2) Drag and drop the prefab from the assets folder onto the public field of the "SpawnAuthoring.cs" script called "PrefabGameObject".
Cheers,
Boris Karavasilev, DOTS QA